The present invention relates generally to durable optical elements. More particularly, the present invention relates to microstructured bearing articles such as, for example, a brightness enhancement film, an optical lighting film or a reflective element.
Microstructure bearing articles, such as, brightness enhancing films, optical turning films or reflective elements, are made in a variety of forms. One such form includes a series of alternating tips and grooves. One example of such a form is brightness enhancement film, which has a regular repeating pattern of symmetrical tips and grooves. Other examples include patterns in which the tips and grooves are not symmetrical and in which the size, orientation, or distance between the tips and grooves is not uniform.
One drawback of current brightness enhancement films and optical lighting films, and the like, is that the tips of the microstructure are susceptible to mechanical damage. For example, light scraping with a fingernail or a hard, relatively sharp edge can cause the tips of the microstructure to break or fracture. Conditions sufficient to break the tips of prior art microstructures are experienced during normal handling of brightness enhancement films, such as, in the manufacturing of liquid crystal displays for laptop computers.
When microstructure peaks are broken, the reflective and refractive properties of the affected peaks are reduced and the transmitted light scattered to virtually all forward angles. Hence, when the brightness enhancement film is in a display, and the display is viewed straight on, scratches in the brightness enhancement film are less bright than the surrounding, undamaged area of the film. However, when the display is viewed at an angle near or greater than the “cutoff” angle, the angle at which the image on the display is no longer viewable, the scratches look substantially brighter than the surrounding, undamaged area of the film. In both situations, the scratches are very objectionable from a cosmetic standpoint, and brightness enhancement film with more than a very few, minor scratches is unacceptable for use in a liquid crystal display.
Durability has been a difficult property to quantify. In the past, durability of microstructure bearing articles has been measured by forming a scratch in the microstructure surface and measuring either the width or depth of the scratch or the gain associated with the scratched microstructure surface. The prior durability tests have not always provided a reliable quantification or a realistic interpretation of how a scratch in the microstructure surface appears as a defect in an optical display.